I just love how the new polite term that people are giving dogs that are DA/HA, which is reactivity.
Personally I like the term "reactive". It's more specific to the cause of the behavior, rather than describing the symptoms like DA/HA is. Yes it is important to know the symptoms, but those terms bring to mind other old training lingo like "alpha" and "dominance", which sadly too many people think is an appropriate way to deal with aggression/reactivity issues. They also scare people (including lawmakers).
Let's say, you have congestion. It's not inaccurate to say "Ado has a runny nose". That doesn't tell us why you have a runny nose, or give any ideas on how to fix it. However, if we say, "Ado has pollen allergies" or "Ado has a cold", we have a much more specific idea of what is causing the symptoms and how to treat the cause. I see "reactivity" vs. "aggression" as the same thing.
To answer Crio's question, Strider used to be happy go lucky around all dogs. Then when he was 1 year old he had a bad week, with three separate incidents of off leash dogs attacking us.
After that he became extremely reactive. If he saw another dog, from 100 yards away he would leap into the air and snarl, roar, etc. I never put him in a position where other dogs got close at that point, but I think if they had gotten close enough he probably would have attacked them. It was a huge problem because he was training to be my service dog, and flipping out like that he definitely couldn't work.
I used the methods in Click to Calm, only I used words as a marker instead of a clicker. Keeping him under threshold, etc. while we reconditioned him to feel good about other dogs again.
In three months, he seemed close to normal. I entered him in a dog show as a test, as it was a safe place (dogs under control) and had a lot of strange dogs. He was perfect, all weekend. Just happy to see them. Then a few months later a great dane attacked him, and he slid backward. Not nearly as bad as before. He wouldn't get outright reactive, but seeing other dogs (esp dogs larger than him) caused him to get stressed, and he would pant and stuff.
We went back and worked some more. Today he is fine with other dogs. He likes playing with them off duty, and ignores them while working. There is another service dog at church, and he is very comfortable laying in a down stay and ignoring that dog from 5 feet away for an hour through a church meeting. Today he went with me to the collie specialty and was totally indifferent/happy with all the hundreds of strange noisy dogs.
My most proud moment lately, was at the grocery store. Someone had a pom with them wearing a SDIT vest. The pom saw Strider and went ballistic, lunging and snarling at the end of its leash. In the past, even on a good day he would have turned around and snarked back at it. I just smiled at him and said, "leave it!" in a cheerful voice. He didn't even look at the pom. He just focused on me and smiled.
I think a huge part of his success was that he started out with a very solid foundation of socialization, good temperament, and training. Because of those things he was able to rebound from those bad experiences pretty well.